1. Field of the Invention
The present invention relates generally to a bearing assembly, and more particularly, to a new and novel cost-effective bearing assembly for use in supporting a water-lubricated propeller shaft such as in large ships, the bearing assembly having low bearing wear and greatly reduced shaft wear.
2. Description of the Prior Art
The need for lower first-cost and longer-lived components in marine propulsion shaft systems is always important because of the cost of acquiring and operating such systems. The challenge that confronts the rudder and main propulsion shaft water-lubricated bearings supplier is how to reduce the total bearing system wear. System wear life reduction concerns shaft/sleeve wear as well as bearing wear. There have been many attempts at producing a bearing with a low coefficient of friction to reduce the bearing wear. However, a low coefficient of friction does not guarantee low system wear. Shaft wear may be excessive while bearing wear is low.
In the prior art, bearing assemblies utilizing a combination of load bearing elements known as staves lining the interior of the bearing housing are well known. The staves have generally been made out of wood or elastomeric materials. Many patents exist for different arrangements of the staves within the bearing housing and materials used for the staves. Each of the various bearing arrangements seek to reduce the coefficient of friction between the bearing surface of the staves and the propeller shaft and to reduce the wear thereon of both members, primarily of the bearing material.
For example, U.S. Pat. No. 2,382,270 issued to Enz relates to a bearing assembly having staves therein and a keeper strip which is formed of laminated fibrous material such as paper, cloth, fabric, fiberglass, etc., impregnated with a resinous binder such as phenolic, urea, vinyl, and the like.
U.S. Pat. No. 3,497,278 issued to Orndorff, Jr., relates to a shaft bearing wherein the bearing surface is formed of strips of elastomeric material which line the inner periphery of a cylindrical housing between radially inwardly projecting lugs with each strip being secured therein by a locking bar inserted into a longitudinal groove in the strip.
U.S. Pat. No. 3,561,830 issued to Orndorff, Jr., relates to a resilient elastomeric insert for bearings of the type used for marine propeller shafts wherein the insert is formed of a low hardness elastomeric material and has rigid backing strips inserted in the elastomer spaced closely adjacent to the bearing surfaces.
U.S. Pat. No. 3,826,547 issued to Finefrock relates to the demountable water-lubricated bearing wherein a housing is formed with a cylindrical bore in which a plurality of symmetrically spaced keys are mounted. A complement of staves formed entirely of elastomeric material are positioned in the bore and directly compressed by a removable compression head to cause tight engagement between the bore and keys. Removal of the compression head allows the staves to assume their unstressed condition in which clearance is provided with respect to both the bore and the keyways.
U.S. Pat. No. 4,607,964 issued to Kramer et al. discloses a water-lubricated bearing assembly having a plurality of circumferentially spaced first set of bearing elements with substantially flat surfaces and water courses therebetween supporting a shaft for rotation. Certain ones or all of such bearing elements support a second set of bearing elements that are movable radially into supporting engagement with the shaft such that either the first set or second set of bearing elements are the prime supports for the shaft and wherein such first and second set of bearing elements have different coefficients of friction.
Heretofore, the material the staves in these bearing assemblies was composed of an elastomeric material such as natural rubber or a synthetic rubber, such as nitrile rubber, or a combination of these materials that could be vulcanized to exhibit both low wear and a low coefficient of friction when lubricated with water. A physical property called xe2x80x9chydrodynamic lubricationxe2x80x9d occurs under certain conditions with bearings made from these materials and lubricated with water. When hydrodynamic lubrication occurs, a very thin film of water is pumped constantly between the bearing surface and the propeller shaft by the rotating propeller shaft, resulting in very low friction between the two surfaces.
The new super demountable designs combine the best features of the demountable designs with the performance advantages of the new Slippery Polymer Alloy (SPA). SPA is the subject matter patented in U.S. Pat. No. 4,735,982 issued to Orndorff, Jr.
There already exists one patent for a bearing assembly using the new Slippery Polymer Alloy (SPA). U.S. Pat. No. 5,932,049 issued to Orndorff, Jr., provides a top layer of SPA provided on the stave element. SPA exhibits superior system wear resistance properties at all shaft speeds.
In a paper entitled xe2x80x9cLubrication in a Sea-Water Environment,xe2x80x9d Naval Engineers Journal (1963), pp. 841-854, the authors commented on judging the performance of propeller shaft bearings and said that static and running friction, noise-producing tendencies and life are very relevant. Cost must be added today in the case of both naval and commercial vessels. Their list of the factors believed to be of concern in bearing and shaft sleeve life are listed below:
1. Load
2. Rubbing Velocity
3. Velocity-Time Interactions
4. Time at Zero Velocity
5. Total Operating Time
6. Shaft and Bearing Roughness
7. Type and Amount of Abrasive
8. Uniformity of Loading
9. Shaft and Bearing Materials
All of these factors must be considered in bearing and shaft sleeve design. For example, the life of most materials is adversely affected by increases in pressure either by adding extra radial load or by shortening of the bearing. When their paper was written, the best water-lubricated bearing designs they considered were made of harder rubber brass-backed staves having machined bearing surfaces and overthick rubber with improper land edges, all of which contributed to unpredictable and intermittent hydrodynamic operation. Excessive time at zero velocity, commonly called dwell time, results in an elevated breakaway coefficient of friction at shaft start-up for many bearing materials. The desired operating time and operating profile (number of start-ups, time spent at low shaft speed) is very important especially in military operations. Shaft and bearing roughness must have the correct finish and texture combination. The possibility of operating in abrasive conditions must be considered. The uniformity of loading is strongly affected by such things as shaft alignment and bearing length, as well as the compressive stiffness of the bearing material. Finally, shaft and bearing materials are listed last, but not the least in importance even when there were only a limited number of suitable material candidates for water-lubricated bearings.
A history of water-lubricated bearings will help to further understand the invention described below.
In 1956 John Penn described to the Institution of Mechanical Engineers his series of experiments running different bearing materials in a tank of water. These experiments led to his invention of the first successful (lignum vitae) water-lubricated bearings. Over the years, various other wood bearings have been developed, such as oil-impregnated maple, which has found a niche in certain types of agricultural machinery where the lubricant is particularly aggressive. Glass bearings were tried as were various ceramic combinations, some of which are still used today. Many types of plastics have been tried with water lubrication over the years. One that has lasted into modern times has been phenolic, a thermosetting plastic reinforced with various natural and manmade fibers. Lignum vitae bearings were in wide use in commercial and international naval vessels until the 1960""s. One of the most well-known applications of lignum vitae bearings was the main shaft bearings for the large number of Liberty and Victory ships constructed in the United States during World War II. After World War II very few water-lubricated bearing materials could satisfy the very stringent friction and wear requirements of the U.S. Navy. Rubber staved (brass backed) bearings did satisfy the requirements for many years. In the 1970""s the brass-backed rubber staves were replaced by softer rubber and plastic (UHMWPE) backed designs of a more efficient design.
In 1944, the elastomer (rubber) had been changed from natural (tree grown) rubber to nitrile, a synthetic elastomer, because some natural rubber staved U.S. Navy ship bearings experienced reversion, which is a melting of the thermoset (cured) compound. This was caused by hysteric heating resulting from repeated dynamic impact loading due to combat damage (bent shafts and broken propeller blades) suffered at the Battle of Midway in June, 1942. Nitrile elastomers do not revert. Naval brass-backed rubber bearings were regarded as too expensive for commercial ship applications due to the complex dove-tail slots and multiple metal shell housings required. In addition, there was a major move starting approximately 30 years ago to adopt sealed oil-lubricated systems for stem tube bearings in large commercial ships. Recently, because of cost and oil pollution problems (all conventional oil seals leak sooner or later) there has been a move back to water-based lubricant systems.
Lignum vitae bearings wear the shafts excessively when abrasives are present and the sources of good timber are scarce today. Fiber reinforced phenolic bearings and polyurethane bearings wear excessively under abrasive conditions and also wear shafts. Polyurethane bearings suffer reversion (melting) under dynamic impact conditions, as did the original natural rubber bearings. In addition, polyurethane bearings suffer from hydrolysis, which is an irreversible softening and cracking when exposed to warm water.
The Romor(copyright), nitrile rubber UHMWPE-backed bearing staves have a reduced breakaway coefficient of friction that is still higher than most of the other non-rubber materials, as disclosed in U.S. Pat. No. 3,993,371. It was discovered that reducing the rubber thickness by 50% and molding a very smooth flat rubber surface reduced both friction and wear of Romor(copyright) bearings by over 90%. Romor(copyright) stave bearings result in a lubrication characteristic in which lubricant-trapping pockets are generated this characteristic is called plasto-elastohydrodynamic lubrication. Rubber bearings also tend to wear the shaft under abrasive conditions, but they still can meet the military specification Mil-B- 17901B (Bearing Components, Bonded Synthetic Rubber, Water Lubricated, U.S. Navy Dept., 1990) requirements. Polyurethane bearings cannot pass the very difficult friction and wear requirements in this specification.
Thin rubber bearings have different wear and friction action from harder nonmetallic materials used in bearings. When a radial load is initially applied to a thin rubber bearing, the deflection is elastic and disappears if the load is removed. After a short period of time under load, the deformation becomes permanent and remains after load removal because of the compression set present in every rubber compound. This causes the plasto-elastohydrodynamic lubrication referred to above. This type of lubrication is enhanced by the special elastic-creep properties of some rubber compounds. As explained above, it was found that thin nitrile rubber bearings for rotating shafts or journals partially immersed in water developed a continuous, unbroken film of water resulting from hydrodynamic lubrication. It was determined that the pressure developed by the hydrodynamic pumping process helped deform the rubber, pushing it out towards the sides and ends of the bearing contact patch, forming a non-contact region or pocket in the center of the contact patch. i.e. plasto-elastohydrodynamic lubrication. It was further found that bearing test sample patches made of polymeric materials which are harder than rubber initially deflected less than the samples made of thin rubber, but eventually developed deep grooves and wore away after partially completing the very severe 28-day procedure (clean wear, or the equally severe 10-hour abrasive wear test).
The assignee of the present application has manufactured and sold for over 25 years the demountable bearing as a cost-effective competitor to conventional molded rubber bearings that have metal or composite shells. The all-rubber demountable bearing is made up of an assembly of two-land staves assembled hand-tight into the housing. The staves are axially longer than the housing. An end ring is bolted onto the end of the housing, axially compressing the rubber staves, which in turn expand circumferentially to tightly lock themselves in place thereby preventing torsional rotation. Elimination of the bearing shell substantially reduces the cost. The very thick rubber wall provides considerable available wear-down thickness. However, the friction torque for the typical demountable bearing is very high because of the concave shape of the lands, the wall thickness and their machined rubber surfaces.
Some of the foregoing material is disclosed in a paper entitled xe2x80x9cNew Extended Life Water Lubricated Bearings, Using a High Performance Polymer Alloyxe2x80x9d by Roy L. Orndorff, Jr., and Richard V. Sheppert, which is incorporated herein by reference.
The present invention takes advantage of the superior wear resistance properties of SPA by utilizing staves made completely from SPA. A plurality of composite staves made from SPA are spaced equidistantly on the circumference of the interior of the bearing assembly housing and aligned in the axial direction of the housing. A rubber stave is placed in between adjacent composite staves to hold them in place. The rubber staves have a thickness somewhat less than the composite staves so that a longitudinal surface channel is formed therebetween. The channels allow water to ebb and flow to the area between the bearing surface of the composite stave and the rotating shaft. Each of the rubber staves is slightly longer than the adjacent composite staves. Upon assembly, the rubber staves extend from the end of the bearing housing. A compression ring is bolted to the end of the housing. As the compression ring is tightened, the rubber staves are axially compressed and forced to expand circumferentially in between the composite staves, tightly locking them in place and preventing torsional rotation.
It is an object of the invention to provide a bearing assembly utilizing a plurality of composite staves and rubber staves, wherein the rubber staves are intermittent with the composite staves to lower the coefficient of friction at all shaft speeds.
It is another object of the invention to provide a bearing assembly that has superior system wear resistance properties of the contacting surfaces of the load bearing members and the contacting portions of the rotating shaft.
It is yet another object of the invention to provide a bearing assembly where the composite material is Slippery Polymer Alloy (SPA).
It is yet still another object of the invention to provide a bearing assembly utilizing a plurality of composite staves and rubber staves placed between adjacent composite staves wherein the rubber staves are less thick than the composite staves so that a longitudinal channel is formed therebetween where ambient water, such as seawater, can flow to lubricate the bearing surfaces of the composite staves contacting the rotating shaft.
It is yet still another object of the invention to provide a bearing assembly utilizing a plurality of composite staves and rubber staves placed between adjoining staves to lock the composite staves in place in the bearing housing.
It is another object of the invention to provide a bearing assembly utilizing a plurality of composite staves and rubber staves placed between adjoining staves wherein the rubber staves are axially compressed by a compression ring to force them to expand circumferentially and lock the composite staves in place.
A still further object is the provision of a stave bearing assembly which aids in grit rejection from the assembly and the shaft with which the assembly is used.
Yet a further object is to provide an improved bearing having no metal parts
An additional object is the provision of a ship bearing assembly where the bearing clearance is easily adjustable.
It is also an object to provide a self-aligning stave bearing structure
It is also another object of the invention to provide an improved staved bearing which can be made using conventional equipment.
Another object is to provide an improved staved bearing assembly having load-carrying staves with lugs that contact a shaft at locations different from the lugs of the other staves.
It is also an object to provide an improved staved bearing structure which can be designed to act hydrodynamically.
Another object is to improve fuel efficiency of a ship or boat.
A further object is to provide better performance of a bearing assembly with increased load.
A still further object is to provide a bearing assembly which eliminates the need for a bearing shell.
An additional object is the provision of a bearing assembly having lower stave installation time and costs.
Still a further object is the provision of a bearing assembly which is capable of water lubrication and for dry runs.
Another object is the provision of a bearing assembly generating low noise at low drive shaft speed.
It is a general object to provide an improved bearing structure for water-lubricated shafts which is economical and efficient in manufacture and use and which is effective in use over a long duration of time.
The foregoing and other objects of the invention are achieved by means of a bearing assembly comprised of a plurality of composite staves arranged circumferentially around the interior of the bearing housing. Both the composite staves and the rubber staves are aligned parallel to the longitudinal axis of the bearing housing. Each of the composite staves is separated from the adjoining composite staves by a rubber stave. The composite stave is made from SPA. Only the composite staves are in contact with the propeller shaft. The rubber staves are less thick than the adjoining composite staves so that a longitudinal channel is formed between adjoining composite staves. This channel provides a means where seawater can enter and exit the bearing housing to lubricate the area between the bearing surfaces of the composite stave and the rotating shaft through xe2x80x9chydrodynamic lubrication.xe2x80x9d